garmin: echo time request reference id

lib/src/garmin.dart

@@ -365,19 +365,29 @@ GarminStatusAck? garminParseStatusAck(GarminGfdiFrame f) {
 /// from.
 const int kGarminEpochOffset = 631065600;
 
-/// Build the full answer to CURRENT_TIME_REQUEST (5052): a RESPONSE (5000)
-/// frame shaped as `ref_msg_type:u16(5052) | status:i8(0) | reference_id:u32(0)
-/// | garmin_timestamp:u32 | utc_offset_sec:i32 | dst_end:i32(0) |
-/// dst_start:i32(0)`. DST transitions are left at 0 — this pass states the
-/// current UTC offset and nothing about a future change to it.
-Uint8List garminBuildTimeResponse({
+/// Build the full answer to a CURRENT_TIME_REQUEST (5052) [request]: a
+/// RESPONSE (5000) frame shaped as `ref_msg_type:u16(5052) | status:i8(0) |
+/// reference_id:u32 | garmin_timestamp:u32 | utc_offset_sec:i32 |
+/// dst_end:i32(0) | dst_start:i32(0)`. `reference_id` echoes the request's
+/// own payload u32 so the watch can match the answer to what it asked. Null
+/// when [request] is not a 5052 frame carrying that id. DST transitions are
+/// left at 0 — this pass states the current UTC offset and nothing about a
+/// future change to it.
+Uint8List? garminBuildTimeResponse(
+  GarminGfdiFrame request, {
   required int nowUnixSeconds,
   required int utcOffsetSeconds,
 }) {
+  if (request.type != kGarminMsgCurrentTimeRequest ||
+      request.payload.length < 4) {
+    return null;
+  }
+  final referenceId =
+      ByteData.sublistView(request.payload).getUint32(0, Endian.little);
   final b = ByteData(23)
     ..setUint16(0, kGarminMsgCurrentTimeRequest, Endian.little)
     ..setInt8(2, 0)
-    ..setUint32(3, 0, Endian.little)
+    ..setUint32(3, referenceId, Endian.little)
     ..setUint32(7, nowUnixSeconds - kGarminEpochOffset, Endian.little)
     ..setInt32(11, utcOffsetSeconds, Endian.little)
     ..setInt32(15, 0, Endian.little)

test/garmin_test.dart

@@ -92,16 +92,29 @@ void main() {
     test('time response carries the Garmin-epoch timestamp and UTC offset',
         () {
       final nowUnix = 1735689600; // 2025-01-01T00:00:00Z
-      final frame = garminBuildTimeResponse(
-          nowUnixSeconds: nowUnix, utcOffsetSeconds: 3600);
+      final request = garminParseGfdiFrame(garminBuildGfdiFrame(
+          kGarminMsgCurrentTimeRequest, [0x2a, 0x01, 0x00, 0x00]))!;
+      final frame = garminBuildTimeResponse(request,
+          nowUnixSeconds: nowUnix, utcOffsetSeconds: 3600)!;
       final parsed = garminParseGfdiFrame(frame)!;
       expect(parsed.type, kGarminMsgResponse);
       final view = ByteData.sublistView(parsed.payload);
       expect(view.getUint16(0, Endian.little), kGarminMsgCurrentTimeRequest);
+      expect(view.getUint32(3, Endian.little), 0x012a,
+          reason: 'reference_id must echo the request');
       expect(view.getUint32(7, Endian.little),
           nowUnix - kGarminEpochOffset);
       expect(view.getInt32(11, Endian.little), 3600);
     });
+
+    test('time response abstains without a request id to echo', () {
+      final short = garminParseGfdiFrame(
+          garminBuildGfdiFrame(kGarminMsgCurrentTimeRequest, [1, 2]))!;
+      expect(
+          garminBuildTimeResponse(short,
+              nowUnixSeconds: 1735689600, utcOffsetSeconds: 0),
+          isNull);
+    });
   });
 
   group('MLR', () {